Covariant formulation of the ponderomotive force

A covariant and gauge-invariant theory of the ponderomotive force is developed for both unmagnetized and magnetized collisionless plasmas. The full expression for the force density exerted on the particle background by the waves (or the ponderomotive force in the general sense) is derived for both the unmagnetized and the magnetized cases. In the magnetized case, the 4-dimensional metric is projected into 'parallel' and 'perpendicular' components. The force density on the particle background then has a similar form to that for the unmagnetised case. The usual ponderomotive force is identified in the same way as in the unmagnetised case and has an equivalent form to that in the unmagnetised case when written in terms of the linear response tensor. 19 refs